This invention relates generally to air handling systems and particularly to laboratory-type hoods.
The accepted method employed for controlling laboratory air contamination and potential exposures of laboratory personnel to toxic, hazardous or radioactive materials is the laboratory exhaust hood (i.e. fume hood). The main purpose of a laboratory hood is to confine air contamination within the hood working area so that contaminant concentrations in the air in the workers' breathing zone outside of the hood face are well below the threshold limit values. Thus it is possible for potentially dangerous experiments and tests to be carried out with relative safety.
Originally, laboratory hoods consisted of a working chamber having an open front and means in the form of a suction fan above the open chamber for drawing air through the opening to carry any undesirable gases or other substances from the chamber. Many of the original hoods also incorporated some type of transparent shield that could be utilized to enclose the opening while the laboratory test was being conducted.
With the advent of air conditioning in most facilities, withdrawing such large amounts of air-conditioned air from the room became extremely costly. As a result (to alleviate some of these problems) supplemental air from the atmosphere surrounding the building has been drawn in and utilized to provide a curtain of air across the hood face. Contamination originating within the hood is not only prevented from escaping the hood into the room by the hood exhaust but is also forced to remain in the hood by a positive supply air barrier. This double protection and clean air supply provide significantly improved protection from hood contamination losses caused by high room air convection and poor hood location. However, in order for an air flow barrier to provide maximum effective worker protection from hazardous hood contamination, the air velocity must be adequate and uniform over the hood face with a velocity vector essentially perpendicular to the plane of the hood face opening. The air velocity at the open face of the hood increases inversely proportionally to the size of the hood face opening as the sash is lowered.
Unfortunately, high face velocities and excessive turbulence are particularly undesirable for several reasons, including contamination of chemicals, equipment, and samples being analyzed, interference with burners and chemical reactions, and uncontrollable loss of toxic or radioactive materials. Safety of the operator is, of course, of paramount consideration.
Thus, there is a continuing need and a continuing search in this field of art for laboratory hood configurations that, for example, improve hood efficiency, especially in regards to worker exposure, and which minimize air curtain velocities in order to maintain a safe working area.